In recent years, from the viewpoint of global environmental issues and crash safety performance, thinner and higher strength structural parts have been sought for automobiles. To meet with these demands, structural parts for automobiles made using high strength steel sheet as materials have been increasing. However, if using high strength steel sheet as the material for press-forming operations to produce structural parts for automobiles, defects such as wrinkles and springback will easily occur in the shaped products. Therefore, it is not easy to use high strength steel sheet as the material for press-forming operations to produce structural parts for automobiles.
As the means for solving such a problem, the technique of hot working the steel material and rapidly cooling it for hardening so as to produce a high strength part has been commercialized. For example, hot pressing works steel sheet at a high temperature where it is soft and high in ductility, so can form complicated shapes with good dimensional precision. Furthermore, by heating steel sheet to the austenite region and rapidly cooling it in a die, it is possible to simultaneously achieve higher strength of the steel sheet through martensite transformation.
However, with such a working method, the steel is heated to a high temperature of 800 to 1000° C., so the problem arises that the surface of the steel sheet oxidizes. If such scale remains, when coating the surface at the next step, the adhesion between the steel sheet and the coating film will become inferior and a drop in the corrosion resistance will be invited. Therefore, after the press-forming, shot blasting or other treatment for scale removal is necessary.
PLT 1 discloses the technique of bending of a metal material during which making a heating device and cooling device move relative to the metal material while using the heating device to locally heat the metal material, giving a bending moment to the portion greatly falling in deformation resistance due to the heating to bend the material to a desired shape bent two-dimensionally or three-dimensionally, and next using the cooling device to cool the material to harden it (below, referred to as “hot 3D bending”).
Hot 3D bending is a working technique used mainly for automobile members. It was developed as a technique for simultaneously meeting the two contradictory needs of lighter weight of the car body and improved crash safety. Hot bending locally heats a steel tube while hardening it by water cooling and simultaneously gives a bending moment to bend the tube and thereby enables production of a complicatedly shaped closed cross-section structural member by a single process. Shaping an auto part in cross-sectional structure at over 1470 MPa becomes possible.
However, this method also heats the steel material to the austenite region and rapidly cools it by a cooling medium so as to try to raise the strength of the steel material by martensite transformation, so there are the problems that oxidation scale forms on the surface, the adhesion of the steel material and coating film becomes inferior when coating the surface in the next step, and a drop in corrosion resistance is invited.
To deal with these problems, the inventors disclosed by PLT 2 an invention relating to a method of production and production apparatus of hardened steel material conveying steel material in its longitudinal direction while heating the conveyed steel material to the hardenable temperature region, then cooling to harden the steel material to thereby suppress or eliminate the formation of oxidation scale.
According to this invention, the steel material is conveyed in its longitudinal direction while using a heating device arranged at a first position separated from the conveyed steel material so as to heat the steel material to the hardenable temperature region and using a cooling device arranged at a second position downstream from the first position in the direction of conveyance of the steel material to spray the steel material with a cooling medium and thereby harden the steel material. During this, at the steel material, an inert gas or reducible gas is filled in the space around the heated part from the heating device so as to produce a hardened steel material suppressed in oxidation scale.